Java virtual machine hardware for RISC and CISC processors

ABSTRACT

A hardware Java accelerator is provided to implement portions of the Java virtual machine in hardware in order to accelerate the operation of the system on Java bytecodes. The Java hardware accelerator preferably includes Java bytecode translation into native CPU instructions. The combination of the Java hardware accelerator and a CPU provides a embedded solution which results in an inexpensive system to run Java programs for use in commercial appliances.

RELATED APPLICATIONS

The present application is a continuation of the application entitled“Java™ Hardware Accelerator Using Microcode Engine,” filed “on Aug. 24,2001, inventor Mukesh K. Patel, et al., U.S. application Ser. No.09/938,886 now U.S. Pat. No. 7,080,362”, which is in turn acontinuation-in-part of the application entitled “Java™ Virtual MachineHardware for RISC and CISC Processors,” filed Dec. 8, 1998, inventorMukesh K. Patel, et al., U.S. application Ser. No. 09/208,741 now U.S.Pat. No. 6,332,215.

BACKGROUND OF THE INVENTION

Java™ is an object orientated programming language developed by SunMicrosystems. The Java language is small, simple and portable acrossplatforms and operating systems, both at the source and at the binarylevel. This makes the Java programming language very popular on theInternet.

Java's platform independence and code compaction are the mostsignificant advantages of Java over conventional programming languages.In conventional programming languages, the source code of a program issent to a compiler which translates the program into machine code orprocessor instructions. The processor instructions are native to thesystem's processor. If the code is compiled on an Intel-based system,the resulting program will only run on other Intel-based systems. If itis desired to run the program on another system, the user must go backto the original source code, obtain a compiler for the new processor,and recompile the program into the machine code specific to that otherprocessor.

Java operates differently. The Java compiler takes a Java program and,instead of generating machine code for a particular processor, generatesbytecodes. Bytecodes are instructions that look like machine code, butaren't specific to any processor. To execute a Java program, a bytecodeinterpreter takes the Java bytecode converts them to equivalent nativeprocessor instructions and executes the Java program. The Java byte codeinterpreter is one component of the Java Virtual Machine.

Having the Java programs in bytecode form means that instead of beingspecific to any one system, the programs can run on any platform and anyoperating system as long a Java Virtual Machine is available. Thisallows a binary bytecode file to be executable across platforms.

The disadvantage of using bytecodes is execution speed. System specificprograms that run directly on the hardware from which they are compiled,run significantly faster that Java bytecodes, which must be processed bythe Java Virtual Machine. The processor must both convert the Javabytecodes into native instructions in the Java Virtual Machine andexecute the native instructions.

One way to speed up the Java Virtual Machine is by techniques such asthe “Just in Time” (JIT) interpreter, and even faster interpreters knownas “Hot Spot JITs” interpreters. The JIT versions all result in a JITcompile overhead to generate native processor instructions. These JITinterpreters also result in additional memory overhead.

The slow execution speed of Java and overhead of JIT interpreters havemade it difficult for consumer appliances requiring local-cost solutionswith minimal memory usage and low energy consumption to run Javaprograms. The performance requirements for existing processors using thefastest JITs more than double to support running the Java VirtualMachine in software. The processor performance requirements could be metby employing superscalar processor architectures or by increasing theprocessor clock frequency. In both cases, the power requirements aredramatically increased. The memory bloat that results from JITtechniques, also goes against the consumer application requirements oflow cost and low power.

It is desired to have an improved system for implementing Java programsthat provides a low-cost solution for running Java programs for consumerappliances.

SUMMARY OF THE INVENTION

The present invention generally relates to a Java hardware acceleratorwhich can be used to quickly translate Java bytecodes into nativeinstructions for a central processing unit (CPU). The hardwareaccelerator speeds up the processing of the Java bytecodes significantlybecause it removes the bottleneck which previously occurred when theJava Virtual Machine is run in software on the CPU to translate Javabytecodes into native instructions.

In the present invention, at least part of the Java Virtual Machine isimplemented in hardware as the Java hardware accelerator. The Javahardware accelerator and the CPU can be put together on a singlesemiconductor chip to provide an embedded system appropriate for usewith commercial appliances. Such an embedded system solution is lessexpensive than a powerful superscalar CPU and has a relatively low powerconsumption.

The hardware Java accelerator can convert the stack-based Java bytecodesinto a register-based native instructions on a CPU. The hardwareaccelerators of the present invention are not limited for use with Javalanguage and can be used with any stack-based language that is to beconverted to register-based native instructions. Also, the presentinvention can be used with any language that uses instructions, such asbytecodes, which run on a virtual machine.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be further understood from the followingdescription in conjunction with the drawings.

FIG. 1 is a diagram of the system of the present invention including thehardware Java accelerator.

FIG. 2 is a diagram illustrating the use of the hardware Javaaccelerator of the present invention.

FIG. 3 is a diagram illustrating some the details of a Java hardwareaccelerator of one embodiment of the present invention.

FIG. 4 is a diagram illustrating the details of one embodiment of a Javaaccelerator instruction translation in the system of the presentinvention.

FIG. 5 is a diagram illustration the instruction translation operationof one embodiment of the present invention.

FIG. 6 is a diagram illustrating the instruction translation system ofone embodiment of the present invention using instruction levelparallelism.

FIGS. 7A-7D are tables showing the possible lists of bytecodes which cancause exceptions in a preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram of the system 20 showing the use of a hardware Javaaccelerator 22 in conjunction with a central processing unit 26. TheJava hardware accelerator 22 allows part of the Java Virtual Machine tobe implemented in hardware. This hardware implementation speeds up theprocessing of the Java byte codes. In particular, in a preferredembodiment, the translation of the Java bytecodes into native processorinstructions is at least partially done in the hardware Java accelerator22. This translation has been part of a bottleneck in the Java VirtualMachine when implemented in software. In FIG. 1, instructions from theinstruction cache 24 or other memory is supplied to the hardware Javaaccelerator 22. If these instruction are Java bytecode, the hardwareJava accelerator 22 can convert these bytecodes into native processorinstruction which are supplied through the multiplexer 28 to the CPU. Ifa non-Java code is used, the hardware accelerator can be by-passed usingthe multiplexer 26.

The Java hardware accelerator can do, some or all of the followingtasks:

1. Java bytecode decode;

2. identifying and encoding instruction level parallelism (ILP),wherever possible;

3. translating bytecodes to native instructions;

4. managing the Java stack on a register file associated with the CPU oras a separate stack;

5. generating exceptions on instructions on predetermined Java bytecodes;

6. switching to native CPU operation when native CPU code is provided;

7. performing bounds checking on array instructions; and

8. managing the variables on the register file associated with the CPU.

In a preferred embodiment, the Java Virtual Machine functions ofbytecode interpreter, Java register, and Java stack are implemented inthe hardware Java accelerator. The garbage collection heap and constantpool area can be maintained in normal memory and accessed through normalmemory referencing.

The major advantages of the Java hardware accelerator is to increase thespeed in which the Java Virtual Machine operates, and allow existingnative language legacy applications, software base, and developmenttools to be used. A dedicated microprocessor in which the Java bytecodeswere the native instructions would not have access to those legacyapplications.

Although the Java hardware accelerator is shown in FIG. 1 as separatefrom the central processing unit, the Java hardware accelerator can beincorporated into a central processing unit. In that case, the centralprocessing unit has a Java hardware accelerator subunit to translateJava bytecode into the native instructions operated on by the mainportion of the CPU.

FIG. 2 is a state machine diagram that shows the operation of oneembodiment of the present invention. Block 32 is the power-on state.During power-on, the multiplexer 28 is set to bypass the Java hardwareaccelerator. In block 34, the native instruction boot-up sequence isrun. Block 36 shows the system in the native mode executing nativeinstructions and by-passing the Java hardware accelerator.

In block 38, the system switches to the Java hardware accelerator mode.In the Java hardware accelerator mode, Java bytecode is transferred tothe Java hardware accelerator 22, converted into native instructionsthen sent to the CPU for operation.

The Java accelerator mode can produce exceptions at certain Javabytecodes. These bytecodes are not processed by the hardware accelerator22 but are processed in the CPU 26. As shown in block 40, the systemoperates in the native mode but the Java Virtual Machine is implementedin the CPU which does the bytecode translation and handles the exceptioncreated in the Java accelerator mode.

The longer and more complicated bytecodes that are difficult to handlein hardware can be selected to produce the exceptions. FIG. 7 is atable-showing one possible list of bytecodes which can cause exceptionsin a preferred embodiment.

FIG. 3 is a diagram illustrating details of one embodiment of the Javahardware accelerator of the present invention. The Java hardwareaccelerator includes Java accelerator instruction translation hardware42. The instruction translation Unit 42 is used to convert Javabytecodes to native instructions. One embodiment of the Java acceleratorinstruction translation hardware 42 is described in more detail belowwith respect to FIG. 4. This instruction translation hardware 42 usesdata stored in hardware Java registers 44. The hardware Java Registersstore the Java Registers defined in the Java Virtual Machine. The JavaRegisters contain the state of the Java Virtual Machine, affect itsoperation, and are updated after each bytecode is executed. The Javaregisters in the Java virtual machine include the PC, the programcounter indicating what bytecode is being executed; Optop, a pointer tothe top of the operand stack; Frame, a pointer to the executionenvironment of the current method; and Vars, a pointer to the firstlocal variable available of the currently executing method. The virtualmachine defines these registers to be a single 32-bit word wide. TheJava registers are also stored in the Java stack which can beimplemented as the hardware Java stack 50 or the Java stack can bestored into the CPU associated register file.

In a preferred embodiment, the hardware Java registers 44 can includeadditional registers for the use of the instruction translation hardware42. These registers can include a register indicating a switch to nativeinstructions and a register indicating the version number of the system.

The Java PC can be used to obtain bytecode instructions from theinstruction cache 24. In one embodiment the Java PC is multiplexed withthe normal program counter 54 of the central processing unit 26 inmultiplexer 52. The normal PC 54 is not used during the operation of theJava hardware bytecode translation. In another embodiment, the normalprogram counter 54 is used as the Java program counter.

The Java registers are a part of the Java Virtual Machine and should notbe confused with the general registers 46 or 48 which are operated uponby the central processing unit 26. In one embodiment, the system usesthe traditional CPU register file 46 as well as a Java CPU register file48. When native code is being operated upon the multiplexer 56 connectsthe conventional register file 46 to the execution logic 26 c of the CPU26. When the Java hardware accelerator is active, the Java CPU registerfile 48 substitutes for the conventional CPU register file 46. Inanother embodiment, the conventional CPU register file 46 is used.

As described below with respect to FIGS. 3 and 4, the Java CPU registerfile 48, or in an alternate embodiment the conventional CPU registerfile 46, can be used to store portions of the operand stack and some ofthe variables. In this way, the native register-based instructions fromthe Java accelerator instruction translator 42 can operate upon theoperand stack and variable values stored in the Java CPU register file48, or the values stored in the conventional CPU register file 46. Datacan be written in and out of the Java CPU register file 48 from the datacache or other memory 58 through the overflow/underflow line 60connected to the memory arbiter 62. The overflow/underflow transfer ofdata to and from the memory to can done concurrently with the CPUoperation. Alternately, the overflow/underflow transfer can be doneexplicitly while the CPU is not operating. The overflow/underflow bus 60can be implemented as a tri-state bus or as two separate buses to readdata in and write data out of the register file when the Java stackoverflows or underflows.

The register files for the CPU could alternately be implemented as asingle register file with native instructions used to manipulate theloading of operand stack and variable values to and from memory.Alternately, multiple Java CPU register files could be used: oneregister file for variable values, another register file for the operandstack values, and another register file for the Java frame stack holdingthe method environment information.

The Java accelerator controller (co-processing unit) 64 can be used tocontrol the hardware Java accelerator, read in and out from the hardwareJava registers 44 and Java stack 50, and flush the Java acceleratorinstruction translation pipeline upon a “branch taken” signal from theCPU execute logic 26 c.

The CPU 26 is divided into pipeline stages including the instructionfetch 26 a, instruction decode 26 b, execute logic 26 c, memory accesslogic 26 d, and writeback logic 26 e. The execute logic 26 c executesthe native instructions and thus can determine whether a branchinstruction is taken and issue the “branch taken” signal.

FIG. 4 illustrates an embodiment of a Java accelerator instructiontranslator which can be used with the present invention. The instructionbuffer 70 stores the bytecode instructions from the instruction cache.The bytecodes are sent to a parallel decode unit 72 which decodesmultiple bytecodes at the same time. Multiple bytecodes are processedconcurrently in order to allow for instruction level parallelism. Thatis, multiple bytecodes may be converted into a lesser number of nativeinstructions.

The decoded bytecodes are sent to a state machine unit 74 and ArithmeticLogic Unit (ALU) 76. The ALU 76 is provided to rearrange the bytecodeinstructions to make them easier to be operated on by the state machine74. The state machine 74 converts the bytecodes into native instructionsusing the look-up table 78. Thus, the state machine 74 provides anaddress which indicates the location of the desired native instructionin the look-up table 78. Counters are maintained to keep a count of howmany entries have been placed on the operand stack, as well as to keeptrack of the top of the operand stack. In a preferred embodiment, theoutput of the look-up table 78 is augmented with indications of theregisters to be operated on at line 80. The register indications arefrom the counters and interpreted from bytecodes. Alternately, theseregister indications can be sent directly to the Java CPU register file48 shown in FIG. 3.

The state machine 74 has access to the Java registers in 44 as well asan indication of the arrangement of the stack and variables in the JavaCPU register file 48 or in the conventional CPU register file 46. Thebuffer 82 supplies the translated native instructions to the CPU.

The operation of the Java hardware accelerator of one embodiment of thepresent invention is illustrated in FIGS. 5 and 6. FIG. 5, section Ishows the instruction translation of the Java bytecode. The Javabytecode corresponding to the mnemonic iadd is interpreted by the Javavirtual machine as an integer operation taking the top two values of theoperand stack, adding them together and pushing the result on top of theoperand stack. The Java translating machine translates the Java bytecodeinto a native instruction such as the instruction ADD R1, R2. This is aninstruction native to the CPU indicating the adding of value in registerR1 to the value in register R2 and the storing of this result inregister R2. R1 and R2 are the top two entries in the operand stack.

As shown in FIG. 5, section II, the Java register includes a PC value of“Value A” that is incremented to “Value A+1”. The Optop value changesfrom “Value B” to “Value B−1” to indicate that the top of the operandstack is at a new location. The Vars value which points to the top ofthe variable list is not modified. In FIG. 5, section III, the contentsof a Java CPU register file, such as the Java CPU register file 48 inFIG. 3, is shown. The Java CPU register file starts off with registersR0-R5 containing operand stack values and registers R6-R7 containingvariable values. Before the operation of the native instruction,register R1 contains the top value of the operand stack. Register R6contains the first variable. After the execution of the nativeinstruction, register R2 now contains the top value of the operandstack. Register R1 no longer contains a valid operand stack value and isavailable to be overwritten by a operand stack value from the memorysent across the overflow/underflow line 60 or from the bytecode stream.

FIG. 5, section IV shows the memory locations of the operand stack andvariables which can be stored in the data cache 58 or in main memory.For convenience, the memory is illustrated without illustrating anyvirtual memory scheme. Before the native instruction executes, theaddress of the top of the operand stack, Optop, is “Value B”. After thenative instruction executes, the address of the top of the operand stackis “Value B−1” containing the result of the native instruction. Notethat the operand stack value “4427” can be written into register R1across the overflow/underflow line 60. Upon a switch back to the nativemode, the data in the Java CPU register file 48 should be written to thedata memory.

Consistency must be maintained between the Hardware Java Registers 44,the Java CPU register file 48 and the data memory. The CPU 26 and JavaAccelerator Instruction Translation Unit 42 are pipelined and anychanges to the hardware java registers 44 and changes to the controlinformation for the Java CPU register file 48 must be able to be undoneupon a “branch taken” signal. The system preferably uses buffers (notshown) to ensure this consistency. Additionally, the Java instructiontranslation must be done so as to avoid pipeline hazards in theinstruction translation unit and CPU.

FIG. 6 is a diagram illustrating the operation of instruction levelparallelism with the present invention. In FIG. 6 the Java bytecodesiload_n and iadd are converted by the Java bytecode translator to thesingle native instruction ADD R6, R1. In the Java Virtual Machine,iload_n pushes the top local variable indicated by the by the Javaregister VAR onto the operand stack.

In the present invention the Java hardware translator can combine theiload_n and iadd bytecode into a single native instruction. As shown inFIG. 6, section II, the Java Register, PC, is updated from “Value A” to“Value A+2”. The Optop value remains “value B”. The value Var remains at“value C”.

As shown in FIG. 6, section III, after the native instruction ADD R6, R1executes the value of the first local variable stored in register R6,“1221”, is added to the value of the top of the operand stack containedin register R1 and the result stored in register R1. In FIG. 6, sectionIV, the Optop value does not change but the value in the top of theregister contains the result of the ADD instruction, 1371.

The Java hardware accelerator of the present invention is particularlywell suited to a embedded solution in which the hardware accelerator ispositioned on the same chip as the existing CPU design. This allows theprior existing software base and development tools for legacyapplications to be used. In addition, the architecture of the presentembodiment is scalable to fit a variety of applications ranging fromsmart cards to desktop solutions. This scalability is implemented in theJava accelerator instruction translation unit of FIG. 4. For example,the lookup table 78 and state machine 74 can be modified for a varietyof different CPU architectures. These CPU architectures include reducedinstruction set computer (RISC) architectures as well as complexinstruction set computer (CISC) architectures. The present invention canalso be used with superscalar CPUs or very long instruction word (VLIW)computers.

While the present invention has been described with reference to theabove embodiments, this description of the preferred embodiments andmethods is not meant to be construed in a limiting sense. For example,the term Java in the specification or claims should be construed tocover successor programming languages or other programming languagesusing basic Java concepts (the use of generic instructions, such asbytecodes, to indicate the operation of a virtual machine). It shouldalso be understood that all aspects of the present invention are not tobe limited to the specific descriptions, or to configurations set forthherein. Some modifications in form and detail the various embodiments ofthe disclosed invention, as well as other variations in the presentinvention, will be apparent to a person skilled in the art uponreference to the present disclosure. It is therefore contemplated thatthe following claims will cover any such modifications or variations ofthe described embodiment as falling within the true spirit and scope ofthe present invention.

1. A method, comprising: decoding and processing both Virtual Machineinstructions and register-based instructions in a CPU having anexecution unit and a register file; wherein the Virtual Machine andregister-based instructions are in a shared instruction cache; andwherein the Virtual Machine stack operands are stored in the CPU'sregister file; wherein for said processing, a common program counterstored in a register in the register file is used for the VirtualMachine and register-based instructions; and wherein in respect of theVirtual Machine instructions, producing register indications for theregister file; and generating a branch taken indication to facilitatethe processing of Virtual Machine and register-based instructions;wherein the branch taken signal flushes a virtual machine instructionpipeline.
 2. The method of claim 1, wherein a top of operand stackpointer for the virtual machine operand stack is stored in the CPUregister file.
 3. The method of claim 1, wherein the CPU performs arraybounds checking for Virtual Machine array instructions.
 4. The method ofclaim 1, further comprising receiving from a branch taken indicator, anindication for flushing at least part of a pipeline associated withprocessing the Virtual Machine instructions.
 5. The method of claim 1,further comprising maintaining counters to keep a count of how manyentries have been placed on the operand stack, for the Virtual Machineinstructions in the register file for the CPU.
 6. The method of claim 5,wherein the operands are moved between the register file and memory dueto at least one of an overflow and underflow condition.
 7. The method ofclaim 1 wherein variables are moved between the register file andmemory.
 8. The method of claim 7, wherein one of operands and variablesare moved between the register file and memory via one of a load andstore operation.
 9. The method of claim 1 wherein the CPU includes aregister to indicate which instructions are to be processed.
 10. Acentral processing unit (CPU) capable of decoding both Virtual Machineinstructions and register-based instructions, the CPU comprising: atleast an execution unit, capable of processing both Virtual Machineinstructions and register-based instructions; a shared instruction cacheto store the Virtual Machine and register-based instructions; a registerfile associated with the CPU to at least store Virtual Machine stackoperands; wherein said processing includes (a) storing a common programcounter in a register in the register file for the Virtual Machine andregister-based instructions; (b) in respect of the Virtual Machineinstructions, producing register indications for the register file; and(c) generating a branch taken indication to facilitate the processing ofVirtual Machine and register-based instructions; wherein the branchtaken signal flushes a Virtual Machine instruction pipeline.
 11. The CPUof claim 10, wherein said processing comprises storing a top of operandstack pointer for the Virtual Machine operand stack in the registerfile.
 12. The CPU of claim 10, wherein said processing comprisesperforming array bounds checking for Virtual Machine array instructions.13. The CPU of claim 10, wherein said processing further comprisesmaintaining counters to keep a count of how many entries have beenplaced on the operand stack, for the Virtual Machine instructions in theregister file.
 14. The CPU of claim 13, wherein said processing furthercomprises moving the operands between the register file and memory dueto at least one of an overflow and underflow condition.
 15. The CPU ofclaim 10, wherein said processing further comprises moving variablesbetween the register file and memory.
 16. The CPU of claim 15, whereinone of operands and variables are moved between the register file andmemory via one of a load and store operation.
 17. The CPU of claim 10,wherein the CPU includes a register to indicate which instructions areto be processed.
 18. The CPU of claim 10, wherein said processingcomprises receiving from a branch taken indicator, an indication forflushing at least part of a pipeline associated with processing theVirtual Machine instructions.